UN refugee chiefs voiced fresh concerns over the viability of the controversial EU-Turkey agreement to deport thousands of illegal immigrants to Turkey after several migrants were hospitalised in riots and hundreds more broke out of a detention centre in Greece.

Three migrants were injured when police used stun grenades to break up a late-night protest on the island of Chios, as frustrations boiled over on the Greek islands where more than 5,000 are being detained ahead of the deportation scheme due to begin on Monday.

Rioting also broke out on Samos, leaving three migrants with stab wounds.

Later hundreds of angry migrants tore down part of the razor wire fence surrounding their holding centre on Chios and began walking to the port in protest, local police officials said.

In scenes reminiscent of the breakout of migrants from Keleti railway station in Budapest last September, video clips on Greek websites showed a column of 700 migrants and refugees, many of them women and children, walking along the tree-lined road to the port.

Migrants pass a banner in a makeshift refugee camp in IdomeniCredit:
AP

Eight migrants were also hospitalised after a fight between Syrians and Afghans in the overcrowded migrant camp at the Greek port of Piraeus near Athens.

The rebellion in the camps is the latest sign that the hard-nosed EU-Turkey plan to deter further migrants returning them directly to Turkey from the Greek islands after fast-track asylum hearings is coming under increasing strain.

The EU had promised to send some 2,000 officials to help implement the controversial scheme, but Greek officials have told The Telegraph that less than 500 had arrived thus far. The UN Refugee Agency (UNHCR) warned that conditions in the holding centres were deteriorating, with growing risks for the migrants.

"The risk of panic and injury in these sites and others is real," said Melissa Fleming, a spokeswoman for the UN High Commissioner for Refugees (UNHCR) in Geneva. “This is in light of continued serious gaps in both countries.” She singled out the islands of Lesbos and Samos, the Athens port of Piraeus and the unofficial refugee encampment at Idomeni on Greece's border with Macedonia as areas of particular concern.

The island detention centres are becoming increasingly overcrowded. On Lesbos, where the majority of refugees and migrants come ashore, the EU’s Moria detention facility now holds more than 2,300 people, with a capacity of 2,000, after arrivals surged this week. Numbers spiked on Tuesday, when 766 refugees arrived, more than twice the island's current daily average.

“If they don’t do something soon, they will have a security problem,” said MSF’s Michele Telaro on Lesbos. “The population in the centres is increasing and conditions are deteriorating. I expect riots any day now.”

Refugees detained inside the centre said there was not enough shelter or food for everyone, with women and small children left to sleep outside on the ground.

“I’m here with my wife and baby. We have to sleep in the open, it’s still cold at night,” said 21-year-old Rohullah, who fled Afghanistan after the Taliban killed his sister in revenge for their father, a policeman, shooting dead a Taliban fighter. “We’re in jail here.”

The rebellion in the camps is the latest sign that the hardnosed EU-Turkey plan to deter further migrants returning them directly to Turkey from the Greek islands after fast-track asylum hearings is coming under increasing strain.

The EU had promised to send some 2,000 officials to help implement the controversial scheme, but Greek officials have told The Telegraph that less than 500 had arrived thus far.

Migrants and refugees arrive at the port of MytilenCredit:
EPA

The UN Refugee Agency (UNHCR) warned that conditions in the holding centres were deteriorating, with growing risks for the migrants. "The risk of panic and injury in these sites and others is real," said Melissa Fleming, a spokeswoman for the UN High Commissioner for Refugees (UNHCR) in Geneva. “This is in light of continued serious gaps in both countries.” She singled out the islands of Lesbos and Samos, the Athens port of Piraeus and the unofficial refugee encampment at Idomeni on Greece's border with Macedonia as areas of particular concern.

The island detention centres are becoming increasingly overcrowded. On Lesbos, where the majority of refugees and migrants come ashore, the EU’s Moria detention facility now holds more than 2,300 people, with a capacity of 2,000, after arrivals surged this week. Numbers spiked on Tuesday, when 766 refugees arrived, more than twice the island's current daily average.

“If they don’t do something soon, they will have a security problem,” said MSF’s Michele Telaro on Lesbos. “The population in the centres is increasing and conditions are deteriorating. I expect riots any day now.”

Refugees detained inside the centre said there was not enough shelter or food for everyone, with women and small children left to sleep outside on the ground.

“I’m here with my wife and baby. We have to sleep in the open, it’s still cold at night,” said 21-year-old Rohullah, who fled Afghanistan after the Taliban killed his sister in revenge for their father, a policeman, shooting dead a Taliban fighter. “We’re in jail here.”

A total of around 51,000 refugees and migrants are now stuck in Greece following this month's closure of the so-called Balkan route, with arrivals more than doubling on Tuesday to 766 from previous days, according to UNHCR figures. The first wave of deportations under the EU-Turkey deal are due to begin on Monday, with the EU border agency Frontex in charge of chartering transport to return the first wave of Syrians, Afghans and Pakistanis back to Turkey.

Several Greek officials with knowledge of the planning told the AP that deportations are likely to start from the island of Lesbos, with migrants from Afghanistan, Pakistan and other countries whose asylum claims are considered inadmissible.

A total of around 51,000 refugees and migrants are now stuck in GreeceCredit:
AP

The transport, the officials said, will be carried out under heavy security escort - with one police minder for every migrant - using buses that will travel from island detention camps and are likely to board straight onto chartered vessels.

The EU-Turkey deal was brokered on March 20 in a last-ditch attempt to break the smuggling chains that saw over 1.1m irregular migrants pour into Europe last year, more than 850,000 of whom came via Greece.

The UNHCR and several international NGOs have voiced serious concerns over the legality of the EU-Turkey deal, withdrawing material support for the scheme which they argue rides roughshod over legal obligation enshrined in the 1951 Refugee Convention.

"The EU must provide greater support, as promised, to boost Greece's creaking asylum system,” the UNHCR spokeswoman said. “Limited hours of registration, daily ceilings on registrations, a lack of access to the Skype system for registration set up by the Asylum Services, are at present adding to the anxiety,” the agency added.

As well as inadequate holding and processing facilities, the UN has also openly questioned whether Turkey can be considered a ‘safe’ third country to which Syrian, Afghan and other refugees can be deported.

Those concerns were highlighted further last night by an Amnesty report which alleged that Turkey had been forcibly returning “hundreds” of Syrian refugees back across its border with Syria in January, in breach of international law.

“The large-scale returns of Syrian refugees we have documented highlight the fatal flaws in the EU-Turkey deal. It is a deal that can only be implemented with the hardest of hearts and a blithe disregard for international law,” the group said.

Turkey, which has taken in 2.7 million Syrian refugees since the civil war began five years ago, denied Amnesty’s characterisation of events.

“None of the Syrians that have demanded protection from our country are being sent back to their country by force, in line with international and national law,” a foreign ministry spokesman told Reuters news agency.